The invention relates to a method for removing contamination on optical surfaces, which are arranged in a vacuum environment, preferably in a projection exposure apparatus for EUV (extreme ultraviolet) lithography, and to an optical arrangement.
In operation, contaminating substances gradually accumulate on the surfaces of optical elements, in particular multilayer mirrors that are used in EUV lithography systems. These substances form, for example, through the reaction of residual gases present in the vacuum environment of the optical surfaces with the EUV radiation to form low-volatile solids. Gases that form such a low-volatile oxidic solid with oxidizing radicals such as OH•, O• arising from the EUV radiation, are for example, sulphur, phosphorus, fluorine or silicon compounds, which may outgas from components (resist etc.) present in the apparatus. Furthermore, gaseous hydrocarbons present in the environment of the optical elements may deposit a layer of carbon on the surfaces.
The formation of a low-volatile solid on the surface of optical elements is highly undesirable, since the substance being deposited gives rise to increased light scattering and absorption. As a result, the optical performance of the overall system in which the optical elements are used declines with respect to transmission, uniformity, stray light and image defects. To reduce the contamination, it is known to bring the optical surfaces into contact with activated hydrogen, i.e. hydrogen that exists in the form of H•, H+ and/or H2+. In this way, on the one hand the carbon is converted into readily volatile hydrocarbons such as methane, and, on the other hand, the oxidic solids compounds can be converted by reduction to readily volatile hydrides.
Currently, to clean optical surfaces in EUV lithography apparatuses by means of activated hydrogen, only methods that cannot be carried out during the exposure operation are used. To clean the surfaces, the exposure operation therefore has to be interrupted, which leads to increased off-times of the apparatus and hence to higher costs. Furthermore, providing and arranging the necessary components for the EUV lithography apparatus cleaning method gives rise to high structural engineering costs. In the case of the known methods for producing atomic hydrogen, such as generation by a heating wire or by means of a plasma, further adverse effects occur, such as the heating of the optical surface or metal deposition on the wire.
A method of removing carbon contamination from optical surfaces disposed in a vacuum chamber is known from US 2004/0211448 A1. In this method, the optical surface is brought into contact with activated gases in an excited energy state. The cleaning gases used are preferably oxygen and carbon dioxide, and alternatively hydrogen, which are activated by electron bombardment, the electrons required for this purpose being obtained by thermal emission from a heating wire.